Effect of molecular shape and electrostatic interactions on the water layer around polar and apolar groups in solution

Abstract

We have carried out neutron diffraction using hydrogen/deuterium isotope substitution and molecular dynamics simulations in order to investigate intermolecular water structure in aqueous solutions of phenol, ethanol, propanoic acid, and glucose. Partial radial distribution functions (rdfs) calculated from both experiment and simulation are compared. These show good overall agreement although the simulation gives more structure than the experimental data in the intramolecular region. From the simulation, the relative number of water molecules coordinated to different chemical groups are calculated and the three-dimensional water density around the different chemical groups on the solute molecule is obtained. For both the experimental and simulated rdfs the intermolecular region is relatively featureless but the simulated water structure shows significant differences between the water coordination around polar and apolar groups. The water is strongly coordinated to the polar groups (OH and COO−), although the hydroxyl groups have fewer close waters than would be possible if all hydrogen bonds were satisfied. There is little evidence for stable water networks around apolar groups. There is also some evidence that the polarized regions of the solutes influence the water coordination around neighboring apolar groups. Apart from the relatively strong polar interactions, the overall shape of the solute molecule is the most important determinant of the shape of the hydration shell. In the case of glucose, the OH groups take part in a well-defined hydrogen-bonded network around the glucose molecule which incorporates the solute molecule in a similar way to water molecules.